This one is easy, because other people have done the heavy lifting. There are several versions in play, depending on what version of D&D is your favourite.
For D&D 3.0 or 3.5, get the Wizards & Warriors genre supplement from the Pinnacle website Downloads section. You’ll also want the d20 to Savage Worlds conversion – what it says on the tin. Used to be on the Pinnacle website, but currently I can only find it at Dragonsfoot in the Savage Worlds section.
For AD&D 1st Edition, go to Dragonsfoot and get Advanced Dungeons & Savages from their Savage Worlds section. This probably works well enough for OD&D as well, though I haven’t tried.
Also at Dragonsfoot: Savage Spells – Basic/Expert D&D spells reworked for SW.
I haven’t seen conversions for 2nd or 4th edition yet, but I’m sure they’re out there.
My experience over the last couple of years is that Savage Worlds works fine for city and wilderness adventures, and in small dungeon complexes, i.e. those with just a couple of levels; but for a full-on old school megadungeon, I think a class and level system such as D&D still works better. Personally, I’m using the races from Wizards & Warriors – sparingly – and making the rest up as I go.
However, and again in my limited experience, if you’re playing a full-on megadungeon, you’re not playing Savage Worlds. Just my $0.02, Your Mileage May Vary.